Pressure coupling in system of water and gas

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GROMACS version: 2023.2
GROMACS modification: No

I’m trying to simulate a system with both water and gas (N2). On the interface sits a protein that acts as a wall between two compartments filled with either solvent or gas, which cannot easily leak through the wall. When I try to enable pressure coupling, the system collapses and the gas compartment gets compressed as if there is no gas there at all (see the gif below). I’m using the oplsaa forcefield with added parameters for N2. The number of gas molecules in the system should be much more then the density at 300K 1 atm would give, meaning there should be a large pressure inside the simulation box, and yet the volume is decreasing.

I ran a simulation with only the gas and it seemed to be stable over a few ns and reach the correct density.

My questions are:

  1. Could I be missing something that would make the gas invisible to the pressure coupling? I’ve never added new parameters to a forcefield before
  2. Can I figure out what pressure just the gas by itself exerts on the simulation box?

I’m not an expert on Gromacs and I have no idea where I could start looking for an explanation for this behaviour. Any help, tips, hints would be greatly appreciated. I can also provide more details, simulation parameters if it helps.

npt_prod_5ns_short

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My guess is that the surface area is so large compared to the volume of the gas that the lowest free-energy state is the collapsed state that you get.